There has been a widely used intercom system capable of confirming a visitor to an entrance from an indoor parent device in a room via an entrance child device located in the vicinity of an entrance door. In recent years, there is a technique attracting much attention, in which a communication function is provided in an indoor parent device and confirmation of visitors is performed from a mobile terminal such as a smartphone that can be carried by a person.
For example, PTL 1 discloses an intercom system for connecting an entrance child device with a mobile terminal via a parent device in response to calling operation by a visitor via the entrance child device and thereafter responding to the visitor from the mobile terminal.